The invention relates to the art of non-volatile electronic memory. It finds particular application where low power consumption, and/or low writing voltage requirements are advantageous. Furthermore, the invention finds application where stored data must not be affected by radiation.
Memory devices play an important role in modern microelectronic systems. For example, memory devices store instruction sets, programs, and/or data for computer processors. Electronic systems use memory devices for example, when performing calculations, signal processing, or data analysis. There are many different memory architectures used for storing information. For example, some moving surface memories store data in the form of magnetic dipoles. Magnetic tapes and discs are examples of these kinds of memory devices. Compact disks store information by varying optical characteristics of points on the surface of the disk. Semiconductor memories typically hold information in the form of charges or electrical potentials in transistor circuits. Transistor based memory devices are inexpensive, relatively small and are compatible with an on-chip addressing circuitry. Therefore, semiconductor memories made of transistors have become the most popular devices for data storage in systems that require a high read/write speed and a compact device size. Nowadays, semiconductor memories find broad applications in areas, which range from computer systems to telecommunications, commercial and military avionics systems, consumer electronics, and advanced weapon systems. In these applications, it is expected that the memories can be accessed at a high speed, exhibit low power consumption, and can be operated at a low driving voltage. Furthermore, these memories must be immune to environmental disturbances, for example, radiation and mechanical shock. While semiconductor memories have achieved many of these goals, their performance is not ideal in some respects. For example, the energy efficiency of some read/write memories is relatively poor, and most transistor memories are sensitive to radiation.
Semiconductor memories are characterized as read-only memory (ROM) and random access memory (RAM). ROM is programmed once, for example, when a machine is being manufactured at a factory or when the ROM itself is being manufactured. From that point onward, data can only be read out of a ROM device. In RAM, data is both written to and read from the device as the requirements of an application dictate. RAM can either be static mode (SRAM) or dynamic mode (DRAM) devices. In SRAM, information is stored, for example by setting up the logic state of a bistable flip-flop. In DRAM, data is stored through the charging of a capacitor. Typically, the information stored in these RAMs is lost if the supply power is turned off. Therefore, these memories devices are called volatile memories. There are memory devices that retain information even after power is removed from them. These devices are known as nonvolatile memories. Nonvolatile memories store information either in a transistor matrix that is connected according to a prescribed mapping relation or in floating gates of MOS transistors. In the latter case, the information stored in a memory cell can be changed by applying an ultraviolet light or an electrical signal to remove charge from or add charge to the floating gate. The floating gate MOS memories in which the contents of the cells can be altered through the use of an electrical signal are called flash memories.
Flash memories are capable of retaining information with the supply power off. Therefore flash memory is widely applied in applications that require low power consumption. Digital cameras, wireless communication apparatuses, computers, as well as many portable electronic systems all use flash memories as their major data storage apparatus. While flash memories have many advantages as nonvolatile memories, their energy efficiency is relatively poor during the programming process. Flash memories write data by injecting charges into floating gates, which are surrounded by dielectric layers used to keep the charges from leaking away. While these dielectric layers are effective in blocking charges from escaping, they also form a high barrier that shields charges from being injected into the floating gate during a data writing process. In the writing/erasing process of flash memories, charges have to penetrate through the dielectrics either by hot carrier injection or by a quantum mechanical conduction process called Fowler-Nordheim tunneling. These injection/tunneling processes generally require a high electric field to help carriers overcome the potential barrier of the shielding dielectrics. The dielectrics are insulators. Therefore, the efficiency of these injection/tunneling processes is poor. For example, in the currently available flash memory technology, the thickness of the dielectrics is in the order of tens of an angstrom. The percentage of charges that can penetrate through these thick dielectrics, to reach the floating gate, is generally lower than 1%. The low efficiency, and the requirement of a high supply voltage, limits the usefulness of flash memories, especially in applications that require a low supply voltage and low power consumption.
In addition to energy efficiency, one of the major drawbacks of semiconductor memories is that they are sensitive to radiation. Traditional semiconductor memories store information in the form of charges or electrical potentials in transistors. These charges and electrical potentials are very sensitive to radiation. In order to prevent the contents of semiconductor memories from being damaged by radiation, special protection layers or device structures need to be applied. However, these approaches typically require more complicated fabrication processes or packaging that introduce a higher cost. The development of a simple, radiation-hard memory is therefore important for many applications. For example aircraft, spacecraft, medical equipment and equipment that, as a side effect of operation, generates radiation all require or can benefit from the use of radiation-hard memory devices.
In order to address the above-described issues, a low power, nonvolatile Microelectromechanical (MEMs) memory cell operative to store data has been developed. This memory cell comprises a first cantilever including a conductive portion, and a second cantilever having an insulated portion and a conductive portion, the second cantilever positioned, at least in part, in overlapping relation to the first cantilever.
A writing process of the memory cell includes a sequential moving or bending and releasing of the cantilevers to place them in a selected orientation. For example, where the first cantilever starts out as an upper cantilever and the second cantilever starts out as a lower cantilever, the writing process comprises moving, bending or flexing the second cantilever out of a movement or bending path of the first cantilever, moving, bending or flexing the first cantilever out of a returning path of the second cantilever, releasing the second cantilever, thereby allowing the second cantilever to follow the returning path of the second cantilever, releasing the first cantilever, thereby allowing the first cantilever to follow a return path of the first cantilever, whereby the first cantilever becomes a lower cantilever and the second cantilever becomes an upper cantilever.
Data or information is encoded in the selected orientation.
The memory cell is used in memory devices. For example, a memory device based on the memory cell comprises a plurality of memory cells, each memory cell comprising a first cantilever, a second cantilever and a cantilever actuator, and a control circuit operative to drive the cantilever actuators to sequentially move or bend and release the first and second cantilevers so as to place the first and second cantilevers in one of a first overlapping relation, and a second overlapping relation.
The memory device is used to make electronic devices. For example, an electronic device that takes advantage of the memory cell comprises at least one memory device, the at least one memory device comprising a plurality of memory cells, each memory cell comprising a first cantilever, a second cantilever and a cantilever actuator. Additionally, the electronic device comprises computational hardware operative to, at least one of, read data from and write data to, the at least one memory device, and an output device operative to present a work product of the computational hardware. For example, the output device can be a xerographic print engine.
One advantage of the present invention resides in the energy efficiency of the writing process of the memory cell. For example, moving or bending the cantilevers can be accomplished through the use of electrostatic force. Therefore, writing can be accomplished by charging and discharging a pair of movable capacitors. The energy efficiency of such a process is much higher than that of, for example, a flash memory.
Another advantage of the present invention is found in the radiation immunity of the memory cell. Data is stored in the physical positions of the cantilevers. The physical positions of the cantilevers are unaffected by radiation.
Yet another advantage of the present invention is that the memory cell can be manufactured in a simple fabrication process that results in a lower device cost. For example, Multi-polysilicon MEMs structures are easier to fabricate than EEPROM (Electrically Erasable and Programmable Random Access Memory) cells, which require very delicate floating gate structures.
Still other advantages of the present invention will become apparent to those skilled in the art upon a reading and understanding of the detail description below.